Besides hardness and texture, probably the most fascinating aspect of gems is their color. There are so many different and wonderful clear and foggy gems with colors that span almost the complete spectrum. Usually it's the impurities and the way they are bound to the mineral crystals that determines the color. For example, NaAl[Si2O6] crystal becomes one type of Jade when laced with chromium, an element that reflects a green part of the light spectrum and absorbs all other colors. Similarly Beryl, which is colorless in its pure mineral form, Be3Al2[Si6O18], becomes Emerald with chromium impurities.

These impurities get absorbed naturally into the crystalline structure of the mineral inside the Earth's crust. The process is stimulated by high temperatures, pressures and sometimes a little help from naturally occurring radiation sources. Clear gems such as diamond or quartz are rarely found colored. However, sometimes these clear gems already contain impurities that can be activated, i.e., the way they bond to the crystalline matrix can be changed by irradiating and heating these gems after they have been excavated.

One can produce bright yellow, blue or green diamonds by irradiating clear diamonds with a radioactive source such as Cobalt 60. Topaz, which is naturally colorless, can be transformed into a cinnamon brown. Fully transparent Quartz can be changed into prized foggy white if it contains enough aluminum impurities or into an Amethyst if it contains enough iron impurities. The problem with gem irradiation is that the final results are not predictable. The gamma radiation from the source basically rearranges the electrons and bonds between the mineral and impurities. Exactly what impurities are present and how they will be rearranged is a probability game. Vibrant colors do however increase the price of the gem significantly, hence most jewelers are willing to take this gamble. If you ever bought a colored diamond, did you ask your jeweler if the color was natural or a result of irradiation?

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About the Author

Anton Skorucak, MSAnton Skorucak is a founder and publisher of ScienceIQ.com. Anton
Skorucak has a Master of Science (MS) degree in physics from the University of Southern California, Los Angeles, California and a B.Sc. in physics with a minor in material science from the McMaster University, Canada. He is the president and creator of PhysLink.com, a comprehensive physics and
astronomy online education, research and reference web site.
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